A cellular or trunked communication system is one in which mobile or portable user terminals, such as mobile telephones or portable or vehicle carried radios or data communication devices, herein collectively referred to as ‘mobile stations’, can communicate via a network infrastructure which generally includes a plurality of fixed base stations (base transceiver stations) and other installations. Each base station has one or more transceivers which serve mobile stations in a given region or area known as a ‘cell’ (or ‘site’) by radio communication. The cells of neighbouring base stations are often overlapping.
Generally, it is desirable for the mobile stations to be served by the base station which can provide the best signals to and from the mobile station. Since mobile stations can move from one region to another it is known for the mobile stations to monitor signals from different base stations, to operate a procedure to determine which base station can best serve the mobile station, to determine whether it would be worthwhile to switch from a current serving base station to another one and if appropriate to carry out such a switch. In the art, the procedure to monitor for and to make determinations regarding a possible switch is known as a ‘cell re-selection’ procedure. The procedure to carry out a switch using such a determination is known as ‘handover’ or ‘handoff’.
Generally, in cellular or trunked mobile communication systems, mobile stations operate independently. When cell re-selection or handover is needed, service may undesirably be delayed or even lost and this could be serious in situations in which it is necessary to maintain communication of information.
EP-A-1401229 describes a group of mobile stations on a moving vehicle such as a train which are communicating with a fixed base station. A controller is provided on the vehicle and operates to assist the mobile stations to undertake a cell handover as a group. Otherwise, the mobile stations operate independently in a normal manner.